Absolutely correct on the liability point. You want to help people, but you can't endanger your craft or crew in the process. There are still lots of ways to help without tying up to a vessel: calling for help, lending tools, giving knowledge... One way you can mitigate if you do chose to tie up is to educate on towing technique and be prepared.
@Julian is absolutely correct (as always) about towing distance. But another consideration: you also have a choice to either tow from the aft or to put your boat along side the disabled craft and 'tow' it with spring lines between the two. That gives you better control (as in
@Speedling 's situation, around docks, etc.). Downside is that it takes longer to hook up and you then have a more difficult time if you need to cut the other boat free in an emergency situation (so if the other craft were taking on water, for example, you would definitely not want to tow along side). If you are going to need fine control of the towed craft, consider spring lines and fenders, with your craft just a bit aft of theirs.